Allow some files specified with source to be optional

See discussion in https://github.com/linuxkit/linuxkit/pull/2027

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

cmd/moby/build.go

@@ -574,6 +574,14 @@ func filesystem(m Moby, tw *tar.Writer) error {
 			if len(f.Source) > 2 && f.Source[:2] == "~/" {
 				f.Source = homeDir() + f.Source[1:]
 			}
+			if f.Optional {
+				_, err := os.Stat(f.Source)
+				if err != nil {
+					// skip if not found or readable
+					log.Debugf("Skipping file [%s] as not readable and marked optional", f.Source)
+					continue
+				}
+			}
 			contents, err := ioutil.ReadFile(f.Source)
 			if err != nil {
 				return err

cmd/moby/config.go

@@ -32,6 +32,7 @@ type Moby struct {
 		Symlink   string
 		Contents  string
 		Source    string
+		Optional  bool
 		Mode      string
 	}
 }

cmd/moby/schema.go

@@ -23,6 +23,7 @@ var schema = string(`
           "symlink": {"type": "string"},
           "contents": {"type": "string"},
           "source": {"type": "string"},
+          "optional": {"type": "boolean"},
           "mode": {"type": "string"}
         }
     },